Radiation system



Fell 12 1957 .1.s. LEVINGER ETAL! 2,731,309

. RADIATION SYSTEM Filed NOV. A22, 1945 .caemos .RAmnTroN sYs'rEMlJosephfevinger, Berwyn,P-a.,fMiloiBsSampson,Santa Fe, i N. Mex.,fArtliur I H. Snell, fk Ridge, Tenn., i and Roger 2G. "Wilkinson, Santa17de, ".NmMexunssgnors-to :the United .-:Stateseof :Americaasfrrepresented byyithe United` States Atomic Energy CommissionApplicnnfNavea-iberz,1945,*sensi`No:nesso li-ciaim. .(cl. 20e-193) eforsome .ye-ars. and. has .beeni extensively stud-iedandiinvestigated. Ingeneral,.it is well-.established that nuclear fission results when thenucleus o`f a'iissionable material, such as the `235 isotope of-4uraniumfcaptures a neutron.

=When such` `capture occurs, the nucleusY of the i fissionable elementor'isotope splits into twonuclearifragments,` comprising lighterelements. Botlr fragments are highly radioactive, and thereis emissionof beta.,an,d`gamma energy as theV fragments decay'to morestalilenulti.A 'Neutrons are also emitted as an'incident toission, andwhenit'is desired to provide. a chain reaction, `-it.is.'nec`essarylthat the numberof neutrons resulting'rfrom fissionfsfi. e.fthereprdductionrate ,of the. system, shall /be';greatcr than unity.Obviously, if this were not so, the system could not continuetochainreact.

Oneimportan and as .yet 'not`flly..explaind,\lect has been observedinconnection wi'thtthe neutron emission occurring as an`in`ciden`tto'fis'sion. Specifically, it has been observed that slightly less than1 per cent of the fission produced neutrons are emitted at periods oftime varying from about 2.5 to about 57 seconds after the fissionactually takes place. These neutrons which are emitted from the fissionfragments subsequent to the actual ssion of the parent nucleus aredesignated as delayed neutrons. So far as is known, the delayed neutronsare emitted from the fission fragment nuclei in a manner substantiallysimilar to radioactive decay. When fission of the 235 uranium isotopeoccurs, about one-third of the total of the delayed neutrons will beemitted as though by radioactive decay of material having a half-life of2.5 seconds. Approximately another third of the delayed neutrons `willbe emitted as by a material having a halflife of 7 seconds. Most of theremainder will exhibit a half-life of 24 seconds, and the final smallfraction, about 4 percentV of the total, will be emitted as by amaterial having a half-life of 57 seconds.,` Y

The present 'invention makes use of this delayed neutron eifect for thepurpose of obtaining neutronA emission in readily controllable densitiesat places remote from the primary source ofthe neutrons, which may be aneutronic reactor or other source of fission-inducing radiation. As willhereinafter appear, the invention makes possible aneutron sourceofunusual flexibility and'of great utility'in the .fields of physicalvresearch and neutron therapy.

An exemplary system for carrying out the invention is diagrammaticallydisclosed in the single gure of the drawings. In that figure, a meansfor inducing fissions in a fissionable material, `which means mayconveniently comprise a source lof neutron radiation such as a neutronicreactor, is indicated at 3. The details ofI a suitable neutronic reactorare given in Fermi et al. Patent No.

nited States Patent 'O ICC 2,108,656,-.whieh,issued onfMay 17, -1-955,on an application-tiled onDeceiber19,1I944. Withinthe reactor 3 .there`is Ypositioned Va.; main reservoir or closure 5 which isconnected to.aclosedliquid circulatingrsystemdisposed externally Aof .the .reactonVThis -systemincludes a ,cir- ,culating pump 1, havin-gA aby 4pass valve3,. a. main control .valve 11, .suitable piping ,13, and ,a secondYreservoir v or .closure 15, which is.of.such afshape anddimensionsthatit maybe employed as aneutronsource, since ,it iswithin .thisportionofrthe -circulating system that.the liberation of delayedneutronsis. caused to take .plface.

-ifftheapparatus isttobe used forthei'apeutic treatment, the: neutronsourcereservoin or.closure15 may convenient- .ly;comprise.,a section.ofgglass .tubing 4adapted to be {disposed .adjacent oreyemwithintherpart of. the'body Vwhich `is tdbesubjectedtoneutronirradiation. .Ifthe :source .is .to be Vemployed f for `physicalr research, the Vdimensions and. arrangements of thereservoir orchamberilS will be determinedfby the particular problenrat hand. When4thesource .oflthezfissionfprodueing raliiation constitutes ajprimarysourceofneutrons, ,such asthe neutronic reactor i :3, `the,second reservoirrportion .15 ,of the circulating `sys- :tern mayproperly be designated as, ar secondary neutron source..It.will.be,undcrstood, however, that means'otherthan.a'.neutronic.reactorcan beusedas the-source of `fission-producingVradiationcomprisirlgaL part of the ap- :paratus of fthe. invention.VForA example, la source. of s high energy deuterons or other`fission-producing radiation could; beV used. Y

.Thefreservoirs Sand 15,.the associated conduitlS, the

control valves 9 .and;11,..and;the pump?, which constituteanclosedliquid.circulatingystem, contain .avsolution or asuspension ofamaterial in which fssions canV be induced 4by the radiation'proVidedrbyithe-t source 3. `Forv example,

.whena .fsourceof neutrons is employed .as-the fission-.prod'ucin'gmeans, the;system.maycontain an aqueous Vsolution of uranyl nitrate, ora suspension ofuraniumoxide in deuterium oxide (heavy water).

During the operation of the apparatus, the solution or suspension offissionable material will be circulated through the main reservoir,which is positioned to receive the fission producing radiation, and thesecond reservoir, which is positioned for use as a neutron source, insequence. In the particular apparatus illustrated, wherein the neutronicreactor 3 is employed as such source, the main reservoir '54 is locatedcentrally ofthe reactor, where the highest neutron density will beencountered. The neutron, or other radiation provided by the source 3,will induce fissions in that portion of the solution or suspension offissionable material contained in the main reservoir 5, and since thismaterial is being continuously circulated from one reservoir to theother, it will be apparent that the circulation rate can be so adjustedby operation of the valves 9 and 11 that delayed neutron emission fromthe iission fragments of the lissions induced in the main reservoir willoccur in the secondary reservoir portion 15 of the external system.VFurthermore, since the delayed neutron emission from the fissionfragments exhibits definite half-life decaycharacteristics following thefission'of the parent nucleus, it is possible to accurately regulate theneutron density at the secondary reservoir 15. Further, ysince verysubstantial time intervals are involved in the delayed neutron emission,it is .possible to pipe the neutron-emitting fluid very considerabledistances.l An additional very important advantage of the inventionresults from the fact that the neutron emission at the secondaryreservoir 15 can be, in effect,.

neutron-emitting fission fragments to the region where the delayedneutron emission is to be used. For example, the main reservoir may befilled with uranium chips, and, water or other liquid may be circulatedtherethrough. This liquid will vpick up the fission fragments from theuranium either mechanically or by dissolution, and will serve to carrythose fragments tothe secondary reservoir where they are to beliberated. Somewhat more stable operation of the apparatus is possibleover extended periods of time by the use of a solution or suspensiombutthe chip arrangement works very satisfactorily, land, in some instances,it is more convenient to assemble and use.

Regardless of which arrangement'is employed, however, the entire liquidcirculating system will soon contain substantial quantities of highlyradioactive fission fragments, and these radioactive fragments willinduce radioactivity in the pump, .the conduit, and the -othermechanical portions of the circulating system. It is extremelydesirable, therefore, that the entire system be enclosed within asuitable shield which may comprise alternate layers 19 of metal, such ascadmium or iron, and layers Z1 of hydrogenous material, such as paraffinor hot-pressed tiberbOard. Such a shield is indicated generally at 17 inthe drawing. It will be understood that the particular shape andproportions of the shield 17 will be determined by the particularrequirements of the installation. Due to this induced radioactivity, itmay be found necessary when the device is being employed for therapeuticpurposes to replace the secondary reservoir 15, which is employed aslthe source portion of the system, at frequent intervals, or to employsuitable shields for protecting the patient from radiation of othertypes than neutron radiation, which may result from the radioactivity ofthe fission products.

The system disclosed comprises a novel and highly useful neutron source.It has proven practical in actual tests, and is a most useful tool inthe fields for which it is adapted. The features of the invention, whichare believed to be new, are expressly pointed out`in the appended claim.f

What is claimed is:

A device for irradiating bodies comprising, a neutronic reactor, aclosed circulating `system including a first reser voir disposed withinthe reactor, a second reservoir, a valve, and a pump for circulating asuspension of uranium oxide in deuterium oxide from the first reservoirto the other within a period of about 2.5 to 57 seconds, a suspension ofuranium oxide in deuterium oxide disposed within the circulating system,and a shield having a first .portion disposed between the reactor andthe section of the circulating system disposed exterior to the reactorand a second portion confronting the circulating system provided with anaperture adjacent'to the second reservoir, whereby the object to beirradiated may be disposed confronting the aperture and the magnitude ofthe neutron flux emanatingfrorn they second reservoir varied by thevalve and pump without altering the neutron ilux within the neutronicreactor.

ReferenesCited inthe le of this patent UNITED STATES PATENTS 549,435caben Novjs, 1895 2,161,985 Szilard June 13, 1939 2,206,634 Fermi et al.July 2, 1940 p kFOREIGN PATENTS 114,150 Australia May 2, 1940 233,011Switzerland Oct. 2, 1944 A861,390, FranceV 0ct. 28, 1940 OTHERREFERENCES Fluegge: .Berlin-Dahlem (from the Kaiser Wilhelm lInstitutefor Chemistry), Die Naturwissenshaften, Sept. 6, 1939, pages 402-410.

ABCD-,3051, Criticality of the Water Boiler and Effective Number ofDelayed Neutrons, by De Hoffmann, Dec. 8, 1944; date dcclassied lan.`30, 1951; 27 pages, U. S. A. E. C.

Smyth: Atomic Energy for Military Purposes, pages 22, 103 104, 177,August 1,945.

Kelly et al.: Phy. Rev. 73, 1135-9 (1948).

